The present invention relates to a multi-beam optical system that divides a light beam emitted from a light source into a plurality of light beams to form a plurality of beam spots on an object surface to be exposed
This kind of multi-beam optical system is used in a scanning photo plotter that forms circuit patterns of semiconductor devices on the object surface, for example. Since a multi-beam scanning photo plotter simultaneously forms a plurality of scanning beam spots on the object surface, a plurality of scan lines can be obtained per one scan, which enables to plot circuit patterns with high speed. In the specification, the scanning direction of the beam spots is defined as xe2x80x9ca main scanning directionxe2x80x9d, a direction perpendicular to the main scanning direction on the object surface is defined as xe2x80x9can auxiliary scanning directionxe2x80x9d, and a line along which the beam spots align is referred to as xe2x80x9ca beam spot linexe2x80x9d.
The multi-beam scanning photo plotter is required to locate a plurality of beam spots along a straight line on the object surface. That is, the beam spot line should be straight. Further, the beam spot line is angled with respect to both of the main scanning direction and the auxiliary scanning direction to make a space between adjacent scan lines be smaller than a distance between adjacent beam spots.
FIG. 9 shows arrangements of beam spots on the object surface in a five-beam scanning photo plotter. The five beam spots should be located at the positions as shown by ellipses illustrated by solid lines that are aligned along a straight beam spot line illustrated by a solid line.
In the field of semiconductor devices, a circuit pattern becomes finer to increase density of a circuit and to downsize a device. A finer circuit pattern requires a high resolution for the scanning photo plotter. Since the resolution increases as the wavelength of the light beam becomes short, it is preferable to use a short wavelength laser beam such as an ultraviolet beam. However, since optical glass generally absorbs an ultraviolet beam, lenses lower the light amount of the laser beam that reaches to the object surface. Therefore, it is preferable that the optical system includes a curved surface mirror instead of a lens when an ultraviolet beam is used.
When the curved surface mirror is used in the optical system, an optical path to be incident on the curved surface mirror and an optical path reflected therefrom should be separated to spatially separate a reflection beam from an incident beam.
However, since the separation of the optical paths bends the beam spot line in the multi-beam scanning optical system, the spaces between scan lines become uneven. For instance, as shown in FIG. 9, the five beam spots are located at the positions as shown by ellipses illustrated by dotted lines that are aligned along the curved beam spot line illustrated by a dotted line.
It is therefore an object of the present invention to provide a multi-beam optical system capable of keeping the beam spot line straight even if the optical system includes the curved surface mirror.
For the above object, according to the present invention, there is provided an improved multi-beam optical system, which includes: a light source; a diffractive beam dividing element that divides the light beam emitted from the light source into a plurality of light beams emerging therefrom at different diffraction angles, respectively; a propagation optical system through which the divided light beams propagate, the propagation optical system including a curved surface mirror; and an image-forming optical system that forms a plurality of beam spots on an object surface by converging the light beams propagated through the propagation optical system.
The curved surface mirror is arranged such that center axes of the light beams reflected from the curved surface mirror are spatially separated from center axes of the light beams to be incident on the curved surface mirror at predetermined separation angles, respectively. This arrangement of the curved surface mirror bends the beam spot line. In the present invention, therefore, the bending of the beam spot line by means of the curved surface mirror is compensated by the arrangement of the diffractive beam-dividing element.
In order to compensate the bending of the beam spot line, the diffractive beam-dividing element is arranged such that the center axis of the light beam to be incident on the diffractive beam-dividing element is inclined with respect to the normal to a diffraction grating surface of the diffractive beam-dividing element. Such an arrangement of the diffractive beam-dividing element causes the bending of the beam spot line. Further, the degree of bending corresponds to an inclination angle of the incident ray with respect to the normal to the diffraction grating surface. Therefore, when the inclination angle diffractive beam-dividing element is appropriately set, the bending of the beam spot line which is caused by the curved surface mirror can be compensated (canceled) by the bending of the beam spot line caused by the diffractive beam-dividing element, which can keep the beam spot line straight.
The diffractive beam-dividing element may be arranged such that the diffraction grating surface is rotated by a predetermined angle about the rotation axis from an initial position where the normal to the diffraction grating surface is parallel to the center axis of the light beam to be incident on the diffractive beam-dividing element. The rotation axis is in parallel with a grating vector of the diffraction grating.
Further, the multi-beam optical system may include a scanning mechanism that scans the beam spots onto the object surface. The propagation optical system may be provided with a multichannel modulator that Independently modulates each of the light beams divided by the diffractive beam-dividing element.